Why domestic water installations must protect water quality, what promotes legionella, and which principles reduce the risk.
Introduction
A water installation must deliver clean drinking water and protect it from contamination. Two key concerns are backflow prevention and the risk of legionella in hot tap water.
Backflow prevention
Contaminated water must never be able to flow back into the drinking water. The required type of protection depends on how polluting the connected medium is (fluid category) — the specific protection device and categorisation are looked up in the applicable rules.
Legionella
- 01Legionella thrives best in stagnant, lukewarm water within a specific temperature range.
- 02Long dead-leg pipes and rarely used draw-off points increase the risk because the water stands still.
- 03Hot domestic water is kept hot enough, and cold water is kept cold enough — the specific temperature requirements are looked up in the applicable guidance.
Principles of execution
- 01Avoid unnecessary dead legs and oversizing that create stagnant water.
- 02Ensure circulation in the hot-water system so the temperature is maintained throughout the whole system.
- 03Insulate cold and hot pipes separately, so cold water is not heated unintentionally.
Fluid categories according to DS/EN 1717
DS/EN 1717 divides all liquids that can come into contact with drinking water into five categories according to how hazardous they are to health. The category increases with risk — from pure drinking water (1) to faecal/microbiologically contaminated water (5) — and it determines how strong a backflow protection is required at the connection.
| Category | Health risk | Typical example |
|---|---|---|
| 1 | None — drinking water directly from the supply | Cold drinking water at the tap point |
| 2 | No health risk, but changed taste, smell, colour or temperature | Heated drinking water (hot service water) |
| 3 | Visible health risk — slightly health hazardous substances | Heating system water without chemical additives |
| 4 | Health risk — toxic/very toxic or cancer-causing substances | Heating system with inhibitor/antifreeze; certain industrial fluids |
| 5 | Health risk — microbiological or viral (faecal) contamination | Wastewater, garden watering, industrial process |
Types of fuses and what they cover
Each category has a minimum safety device type. The safety devices are grouped into families (A–L) by mode of operation — from the physically safest free-air gap to a simple check valve. The higher the category, the more robust and controllable a safety device is required.
| Fuse type | Principle | Covers up to category |
|---|---|---|
| AA / AB | Free air gap (physical disconnection) | 5 |
| BA | Controllable backflow prevention with pressure zones (RPZ) | 4 |
| CA | Non-controllable securing with differential pressure zones | 3 |
| DA / DB | Vacuum/aeration valve (by type) | 3 (according to type) |
| EA | Controllable check valve with test point | 2 |
| EB | Ordinary check valve (limited application) | 2 |
| HA / HD | Hose coupling/drain protection (by type) | after type |
Fluid categories and protection types follow DS/EN 1717 as set out in Rørcenter guidance 015 (Technological Institute). Note the logic: an RPZ valve (BA) with controllable zones is required for toxic media (cat. 4), while only a physical air gap (AA/AB) is safe enough for faecal contamination (cat. 5). (Source: DS/EN 1717; Rørcenter guidance 015.)
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